1. Field of the Invention
The present invention relates to a procedure and device for overload and short-circuit protection of output drivers.
2. Description of Related Art
The output drivers (transistors, FETs etc.) controlling the electric power supplied into a load will break down if overloaded or short-circuited. When the load is shortcircuited or when it assumes a low resistance (e.g. one of load solid state switches has breakdown), the collector current of the driver increases at inadmissible values. The voltage across the driver increases also which results in a high power dissipation and consequent destruction of the driver. In order to limit the collector current to an admissible value, in many devices for protection of output drivers, a resistor is connected in series in the load circuit. However, since the resistor is power dissipating and the drop of potential across it is important, this solution cannot be implemented in some applications. Furthermore, because the output drivers must be protected instantly when the faulted situation appears, a device capable to early detect an increase in collector current and with a quick protective response is required. A device for overload and short-circuit protection is proposed in U.S. Pat. No. 4,695,915. This device however, does not use a reference value, which means that the overload current limit cannot be set to a desired value. Besides, the current limit varies when the voltage across the load, varies due to the variations in the output transistors base current. Also, the output transistor requires a large control current, resulting in large thermal losses. For this reason it is practically impossible to integrate several output drivers in a single IC. Moreover, the device is not turned on immediately when the control signal is applied. Instead for this purpose, a separate external pulse is required.